Free space optical communications (FSOC) operate on a similar principle to fiber optics-based communications. Light propagating in free space can be used to transmit data between two points using a modulated light source. The technology can be useful in situations where the physical connection of the transmit and receive locations is difficult, for example in cities where laying of fiber optic cables can be expensive. FSOC can also be also used to communicate between space-craft, because outside of the atmosphere there is little to distort the signal. Current optical links typically use infrared laser light, although low-data-rate communication over short distances is possible using LEDs. Transmission distances up to the order of 10 km are possible with current technology, but the distance and data rate of these connections are highly dependent on atmospheric conditions.
When used in a vacuum, for example for inter-space craft communication, FSOC can provide similar performance to that of fiber-optic systems. However, for terrestrial applications, various factors, including beam dispersion and absorption in a non-vacuum atmosphere and scattering and/or absorption by particles such as rain, fog, snow, dust, aerosols due to pollution or natural processes, and the like can attenuate or otherwise degrade the quality of a received beam, thereby leading to a higher bit error ratio (BER) in the transmitted data. In some cases, atmospheric conditions like fog, clouds, or high aerosol concentrations can even completely break a connection. Current solutions to these issues include multi-beam or multi-path architectures that use more than one sender and more than one receiver. A larger “fade margin” or extra transmission power that is reserved for challenging atmospheric conditions such as rain, smog, fog, smoke, or the like, can also be used.